It's not the quantity has much to do with quality and usability, it's that quantity has a lot to do (clearly) with marketshare. The Motley Fool does a pretty good job explaining why the sheer number of manufacturers and models is going to continue to drive growth in Android marketshare:
http://www.fool.com/investing/high-growth/2010/05/27/deciphe...I look at it like this. For years usability and quality difference between DOS (and later Windows) machines and Macs was like night and day. Particularly if you look at the early days of Windows, it was unbelievably primitive compared to Macs of the same era. And yet, because of the number of manufacturers and models of DOS/Windows PC's and the number of sales channels those PC's where sold through, the sales volume for PC's has always dwarfed the sales volume for Macs.
If you look at the Motley Fool's analysis, it's clear that Android will have the same advantages of diversity of models, distribution, etc. over iPhone that PC's have traditionally had over Macs. However, IMHO the situation is even worse for Apple this time, as the quality and usability of the end user experience between iPhone and Android much closer than it was between Mac's and DOS/Windows PC's.